This invention relates to the manufacture of flat glass by a process of floating glass on molten metal while forming it into a continuous sheet or ribbon of glass. More particularly, this invention relates to improved methods of controlling such processes.
In the various methods of making flat glass by floating it on molten metal during forming, molten glass is delivered onto the surface of a bath or pool of molten metal, such as tin, and this glass is formed into a dimensionally stable, continuous sheet or ribbon of glass by advancing it along the surface of the pool of molten metal while cooling it and applying attenuation forces to it. Following formation of a continuous sheet of glass, the glass is removed from its supporting molten metal and conveyed to and through an annealing lehr to anneal the glass. The glass is then cut into pieces of selected size for further processing and sale. See U.S. Pats. No. 710,357, No. 789,911, No. 3,083,551 and No. 3,843,346 for descriptions of several processes for making flat glass to which this improvement may be applied.
Flat glass made by any of these processes may be made in a wide range of thicknesses and widths and at substantially diverse rates depending upon the size of a chamber in which it is produced and upon the thermal and mechanical force conditions imposed upon the glass while it is being supported and formed on molten metal. Under equilibrium conditions and in the absence of accelerating or attenuating forces acting on the glass, a sheet of glass having a thickness of about 6 - 7 millimeters is produced. This is characterized as an "equilibrium thickness." Glass may, of course, be made having a thickness that is greater or less than equilibrium thickness.
In general, to obtain a final glass thickness which is greater than equilibrium thickness, it has been the practice to place restrictor elements on either side of the ribbon of glass at a point and at a location within the bath chamber where the glass is sufficiently hot to behave as a viscous liquid. In order to make glass having a thickness which is less than equilibrium thickness, there have been practiced two alternative techniques: one technique is to discharge the molten glass onto a tin bath at a discharge rate which produces a relatively wide body of molten glass and to apply attenuating forces to the glass along its direction of travel by substantial acceleration of the ribbon due to high speed operation of the driving rolls and lehr rolls downstream in the process; a second method is used to discharge glass at a sufficiently high discharge rate to provide for a body of molten glass which is substantially wider than the intended final ribbon width and to apply lateral attenuation forces to the ribbon by edge rolls or the like, in addition to supplying attenuation forces in the direction of ribbon travel so that the ribbon is steadily made narrower as its thickness is reduced.
In the making of equilibrium thickness glass and thinner than equilibrium thickness glass by the float process, substantial attenuation of the ribbon causes a decrease in ribbon width corresponding to travel downstream through the bath chamber. Though the effect is less pronounced in making thicker glass, some attenuation occurs in its manufacture also. A variety of forces and properties affect the ultimate thickness and width of a ribbon of flat glass. Among the forces affecting glass width and thickness are the axial or longitudinal attenuating forces which are imposed by the pulling of the cooled, rigid glass passing out of the float bath and through the annealing lehr, the lateral or sideward pulling or pushing of edge attenuators or restrictors, respectively, the pushing force of additional molten glass being discharged from the melting and refining furnace into the float bath and the downward forces of gravity and atmospheric pressure acting upon the floating ribbon of glass. Properties which affect the ultimate width and thickness of a float-formed glass ribbon include the density of the glass, the surface tension of the glass-metal interface, the viscosity of the glass and the variation of these properties and the flow conditions of the underlying metal due to thermal effects and variations existing throughout the float bath chamber.
Ordinarily, a continuous ribbon or sheet of glass, after being annealed and cooled, is cut into useful piece sizes. By a series of tranverse cuts, the continuous sheet is partitioned into a plurality of large discrete sheets known as "uncuts" or "lehr ends." Marginal edges of the sheets are ordinarily cut away or trimmed from the sheets and returned as cullet to the glass melting furnace. The trimmed glass sheets are then cut into smaller pieces of desired sizes for sales and ultimate use. Desirably, the width and length of each trimmed sheet is such that a combination of desired size pieces may be cut from it without having excessive waste or trim. If the width of the sheet of glass is not precisely controlled, it is necessary to operate with a target width that is sufficiently greater than that required to yield no waste or trim so that narrowing variations in width can occur without yielding a ribbon too narrow to permit cutting a discrete number of desired sizes from it since waste will sharply increase if fewer pieces must be cut from the sheet.
With respect to the thickness of the glass, any given end use of glass has some specified glass thickness and in order to accommodate occasional uncontrolled variations in thickness such a specified thickness will admit to a variation about the specified thickness within some acceptable range. To the extent that the glass thickness can be more precisely controlled, it is possible to operate with a target thickness closer to the acceptable minimum thickness. This serves to save manufacturing costs as less glass needs to be melted, refined and formed to yield a commercially equivalent amount of glass. For example, if glass having a nominal commercial thickness of 7/32 inch (5.5 mm) is permitted by commercial standards to have any thickness, it is considerably more efficient to have a production target that is the nominal thickness less two thousandths inch rather than the nominal thickness itself. This lower production target is only feasible if control is sufficiently precise so as to insure that uncontrolled variation will not exceed three thousandths inch rather than a less precise control of five thousandths inch which would require a production target identical to the nominal thickness. Such an apparently small change can be economically significant, for in a facility making about 400 tons of glass per day each saving of one thousandths inch of glass thickness yields a capacity potential for producing glass at a rate of more than 1000 additional square feet of glass per day
The precise control of glass thickness and width have, thus, been matters of considerable interest and importance in the art of glassmaking. Exemplary patents illustrating this are U.S. Pat. No. 3,531,274 to Dickenson et al and U.S. Pat. No. 3,764,285 to Matesa et al. In general, the control schemes of the past have been successful in terms of controlling the variable, thickness or width, which has been the particular objective of each control scheme. It has been recognized that each control scheme is conveniently operated to ignore certain realities in order to provide simple and reliable control of the variable over which control is desired. Thus, it has been convenient to precisely control the thickness of a continuous sheet of glass by controlling thermal and attenuating force conditions in a forming chamber and permitting some variation in the width and speed of the sheet of glass to accommodate the thickness control. It has alternatively been convenient to control the width of a continuous sheet of glass by varying the rate at which molten glass is delivered for forming while permitting some variations in glass thickness and speed to accommodate the width control.
It is an objective of this invention to precisely control the mass throughput of a glassmaking process so that both the thickness and width of the continuous sheet being produced can be precisely controlled to strict tolerances. As a result of practicing this invention loss of production capacity and waste of glass due to varying and excessive trim and cutting losses and due to excessive glass thickness are substantially reduced.